Dicamba resistance in pigweed selected in research greenhouse, not in the field

By Mary HightowerU of A System Division of Agriculture

Fast facts:

Dicamba resistant pigweed not found in fields

Resistance development in lab only

Pigweed has strong tendency to evolve resistance

(590 words)

NEWPORT, Ark. – University of Arkansas System Division of Agriculture researchers
have selected for dicamba-resistant pigweed, documenting how genetic resistance develops
and how the industry must work to protect the few remaining weed-fighting options.

Bob Scott, an extension weed scientist, emphasized that the finding was the result
of controlled greenhouse studies and not a confirmation of anything found in any field.

PIGWEED -- Pigweed pokes its head above a soybean field in this 2009 file photo.
(UofA System Division of Agriculture image)

“Through experimentation in the greenhouse, we selected for a population of pigweed
that is tolerant to the herbicide dicamba at a field rate,” Scott said Monday. “This
pigweed population was not found to be resistant to dicamba in nature or in any field.”

Controlled greenhouse experiment

Scott’s colleagues, division Weed Scientist Jason Norsworthy, Norsworthy’s post-doctoral
associate Parsa Tehranchian and Stephen Powles, professor-plant biology at the University
of Western Australia, designed the greenhouse experiment to examine the potential
for the future of resistance.

The researchers began with dicamba-susceptible pigweed collected from the field. The
researchers exposed three generations of pigweed to sub-lethal doses of dicamba, “which,
of course, is a recipe for resistance development,” Scott said.

The first two generations were still susceptible to dicamba. The third generation
was now resistant. Scott emphasized that this resistant population is highly controlled
and will never be released to the field.

Protecting what works

“The research that was conducted in the greenhouse that resulted in a dicamba-resistant
pigweed illustrates how multiple resistances have developed,” Scott said. “One pesticide
quits working and so we replace it with another, and so on and so on, until you are
left with a weed population or insects for that matter that can tolerant multiple
modes of action.

“This is the inevitable result of using a single effective mode of action to control
a given pest,” he said. “As new technologies emerge we must be diligent in our efforts
to stop this cycle”

It was pigweed’s emerging resistance to glyphosate that drew international attention
to the issue of herbicide resistance. With glyphosate out of the picture as an effective
control on a growing number of farms, soybean and cotton farmers looked to dicamba
to manage the notoriously difficult weed with the release of Xtend soybean and cotton.
The Xtend crops can tolerate dicamba deployed to control weeds.

Scott said the greenhouse findings plus the lack of emerging control options paint
a frightening picture for growers.

The Tweet

News of the findings, which will be presented next month at the Weed Science Society
of America/Southern Weed Science Society annual meeting, came out on Twitter over
the weekend.

The 78-character tweet turned into scores of phone calls on Monday to Scott, Norsworthy
and colleague Tom Barber, also a weed scientist with the Division of Agriculture.

“I think between the three of us we fielded at least 100 calls,” Scott said Monday
night. “I even had farmers asking whose field we found it in, and of course this was
a greenhouse-only finding.”

Multiple resistance

For farmers, the number of tools they have for weed control is dwindling quickly.
(See “Loss of Enlist Duo cuts already thin weed-fighting arsenal for bean farmers,” http://bit.ly/EnlistDuoArk2015) And it’s not just that the pigweed is resistant to one class of controls, but “some
of these populations have now been found to be resistant to as many as four formerly
effective modes of action,” Scott said.

The researchers, in their conclusion said their finding “strongly suggests that there
will be sizeable evolutionary consequences if dicamba is not properly stewarded in
dicamba-resistant crops such as applying it repeatedly in a manner that provides less
than complete control.”

The University of Arkansas System Division of Agriculture offers all its Extension
and Research programs and services without regard to race, color, sex, gender identity,
sexual orientation, national origin, religion, age, disability, marital or veteran
status, genetic information, or any other legally protected status, and is an Affirmative
Action/Equal Opportunity Employer.

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Media Contact: Mary HightowerDir. of Communication ServicesU of A Division of Agriculture(501) 671-2126mhightower@uaex.edu